The intensive use of chemicals, and complicated production processes, makes the production of ferrates challenging and has kept a widespread application of ferrates limited. Ferrates and the materials and processes, used in the making of ferrates are well known. The ferrate ion FeO42− [Oxidation State +6], is well known as the ferrate ion. The ferrate ion, FeO42 is known to be a tetrahedral ion that is isostructural with chromate or permanganate. The ferrate ion has been shown to exist as the tetrahedral species FeO42−. Redox potentials for this ion have been estimated in both acidic and basic media. For example, ferrates, and the processes for producing ferrates are shown and disclosed in U.S. Pat. Nos. 7,476,324; 6,790,429; 4,304,760; 4,405,573.
Iron commonly exist as metallic iron in (Fe(0)), ferrous (Fe(II)), and ferric (Fe(III)) forms in the natural environment. Examples of ferrous and ferric oxides including wuestite, hematite, magnetite, and goethite are shown in (Table 1). Sodium and potassium salts of higher oxidation states of iron ranging from +4 to +8 have also been prepared (See FIG. 1, Table 1).
Salts of ferrate(VI) have been of great interest because of their role as oxidants and hydroxylating agents in industrial and water treatment processes, such as the development of a “super iron” battery, green chemistry synthesis, and non-chlorine oxidation for pollutant remediation.
Ferrate(VI) [oxidation state of iron +6] provides an environmentally benign high energy density cathode for batteries. Selective oxidation by ferrate(VI) can be utilized in synthesizing organic compounds without producing toxic by-products. Ferrate(VI) can complete oxidation reactions in shorter time periods than oxidations carried out by other oxidants such as permanganate and chromate. Iron, unlike chromium and manganese, is considered non-toxic; therefore, ferrate(VI) can make industrial processes more environmentally benign by achieving cleaner technologies for organic syntheses.
Ferrate(VI) is an emerging water-treatment disinfectant and coagulant, which can address the concerns of disinfectant by-products (DBPs) associated with currently used chemicals such as free chlorine, chloramines, and ozone. Like ozone, ferrate(VI) does not react with bromide; so the carcinogenic bromate ion is not produced in the treatment of bromide-containing water. Ferrate(VI) also acts as a strong oxidant to degrade a wide range of compounds present in wastewater and industrial effluents. Importantly, ferrate(VI) has the ability to oxidize emerging contaminants like estrogens, bisphenol-A, and pharmaceuticals present in water.
Ferrate(VI) can achieve disinfection at relatively low dosages over wide ranges of pH. The results suggest irreversible inactivation of Escherichia coli (E. coli) by ferrate(VI). Treatment of water sources collected worldwide by ferrate(VI) have demonstrated more than 99.9% kill rates of total coliforms. The results have shown that ferrate(VI) can inactivate E. coli at lower dosages or shorter contact time than hypochlorite.
Disinfection tests of sodium ferrate(VI) on spore-forming bacteria demonstrated that aerobic spore-formers are reduced up to 3-log units while sulfite-reducing clostridia are effectively killed by ferrate(VI). Both bacteria are resistant to chlorination. Ferrate(VI) can also be effective in treating emerging toxins in the aquatic environment. Ferrate(VI) is also an efficient coagulant.
Examples include removal of metals, nutrients, radionuclides, and humic acids. Ferrate(VI) is advantageous in coagulation where it can be applied in a pre oxidation step of the treatment. Importantly, the multi-functional properties of Ferrate(VI) can thus be utilized in a single dose for recycling and reuse of water and wastewater.
A number of alkali and alkaline earth salts of ferrate(VI) have been synthesized. Three main approaches include: (i) dry thermal synthesis, (ii) electrochemical synthesis, and (iii) wet chemical synthesis. The wet method has been used to prepare sodium and potassium salts of ferrate(VI) (Na2FeO4 and K2FeO4). An iron(III) salt was oxidized by alkaline hypochlorite.2FeCl3+3NaOCl+10NaOH→2Na2FeO4+9NaCl+5H20
The reaction will produce soluble Na2FeO4, containing high levels of NaOH and unused NaOCl. Thus sodium ferrate(VI) solution produced by above stated method has limited practical applications. The use of sodium hypochlorite in the reaction does not qualify it to be an environmentally-friendly method.
Ozone and OXONE™ (a mixture of K2S04, KHS04, and KHSO5 can be used instead of hypochlorite to synthesize ferrate. However, the intensive use of chemicals in wet chemistry is not recommended for industrial production. In the literature, a solid compound of ferrate(VI) as potassium ferrate(VI) (K2FeO4) has been produced from soluble Na2FeO4 by adding potassium hydroxide (KOH). The syntheses of K2FeO4 using this procedure involved several steps to make the solid product of ferrate(VI) cumbersome with very low economic feasibility.
For electrochemical Ferrate production, iron or iron salts are applied as an anode in an electrochemical cell containing high concentrations of hydroxide ion, typically more than 10 M OH−. The reaction below is shown as one example of synthesizing Ferrate from iron rod.Fe+8OH—FeO42+4H2O+6e 
However, there exist drawbacks and disadvantages associated with the electrochemical synthesis of ferrate, including, for example, the formation of a residual passive film on the electrode surface and the extent to which the competitive oxygen evolution reaction (OER) is present, at the ferrate formation potential (to ambient temperature), reduces the efficiency of ferrate synthesis process.
Efficiency for the ferrate(VI) generation is sensitive to electrode pretreatment and to the reaction conditions. For that reason, attention is directed to chemical composition, geometry, mode of activation of the anode, and to the electrolyte composition. The influence of other experimental parameters, for example, temperature, applied current, and electrolysis time, is considered in the process of ferrate production.
As would be well known to those skilled in the art, efficiency in the industrial production of ferrate(VI) is compromised where control over the scope or range of variable parameters, is required. As is well known to those skilled in the art, electrochemical methods produce sodium ferrate(VI), with a high level of hydroxide ion and have limited value for application in treatment of water and wastewater. Furthermore, the use of KOH for the production of paste of K2FeO4 for applications will also have similar limitations. The separation of solid/dry K2FeO4 will involve numerous processes; causing economic disadvantages.
In the thermal method of ferrate(VI) production, ferric salt and oxidant are heated to a high temperature. For example, the heating of ferric oxide and potassium nitrate is heated to more than 1000° C., to generate solid potassium ferrate(VI). However, the requirement for a high temperature is a safety and an energy concern. Additionally, solid salts of ferrate(VI) must be protected from ambient humidity, which accelerates decomposition of the ferrate(VI) to Fe(III). Decomposition in ambient environment, can happen in several hours, leading to half-life concentrations.